1. Field of the Invention
The present invention generally relates to communications systems and more particularly to a method and system for improving tracker performance in code division multiple access (CDMA) communication systems.
2. Description of Related Art
The IS-95 standard defines features of what is frequently called a second generation code division multiple access (CDMA) communication system, a type of Direct Sequence spread spectrum modulation. An IS-95 system communicates information from a base station to mobile stations over a series of traffic channels. These traffic channels transmit and receive information, spread with a traffic channel pseudonoise (PN) code, unique to each mobile station. Using precise timing and phase information derived from a pilot channel, the mobile station is able to acquire a setup channel, and eventually, the overall system time. With this system time, the mobile station is able to differentiate between base stations and synchronize its demodulation circuitry with sufficient accuracy to recover the received traffic channel message.
In a third-generation Universal Mobile Telecommunication System (UMTS), base stations are not closely synchronized. Although mobile stations attempt to lock the base station carrier frequency, the operation is imperfect such that a frequency offset is present between a mobile station and a corresponding base station. The frequency offset becomes more obvious when the mobile station is in a handoff mode (a handoff mode is where a mobile station talks to two unsynchronized base stations) or the mobile station moves at a higher speed, which causes higher Doppler frequency. Without performing some type of timing error correction, an on-time error in a base station receiver can increase with time due to this aforementioned frequency offset. An on-time error is a timing error that represents a time difference between a detected transmission path, or a finger, and the actual transmission path.
To combat and correct these on-time errors, base station receivers employ what is called a tracker. In the case where a frequency offset exists between a mobile transmitter and base station RAKE receiver, a finger (e.g., detected propagation path from mobile station transmitter to base station receiver) in the base station receiver experiences a certain adjustment in slew rate. The slew rate can be defined as a finger timing change rate in a chip per radio frame. The slew rate due to the frequency offset is such that an ideal on-time (i.e., ideal finger timing) drifts over time. Thus, a tracker should be able to track the timing drift and maintain on-time error as small as possible.
Accordingly, a tracker is needed for each finger in the RAKE receiver of the base station to track the correct timing of a specific finger. Due to a Doppler effect and due to the above-noted frequency offset between the transmitter and the receiver, the timing of a finger may drift over time. The tracker is supposed to constantly lock on a finger's timing so that most of signal energy in that finger can be utilized by the receiver.
FIG. 1 illustrates a block diagram of a tracker typically used in an IS-95 system, and is referenced to explain a conventional tracker algorithm. In the tracker 100 of FIG. 1, an interpolator 110 receives samples at a rate of 2 samples/chip for each frame of data from sample buffer 105 and, via a filtering action performed in interpolator 110, increases the resolution to output samples of a frame at a rate of 16 samples/chip. These samples collect in a circular buffer 120 until samples for an entire frame of data have been stored in circular buffer 120.
Accordingly, given an on-time value, the tracker 100 reads two samples per chip that are output from the circular buffer 120. One sample, which is called the “early sample”, is at a timing of on-time, minus ½ chip period and the other sample (the “late sample”) is retrieved at a timing of on-time plus ½ chip period. These samples accumulate in an early accumulator 122 and in a late accumulator 124, respectively as the circular buffer 120 is filling up, wherein samples of an entire frame of data fill circular buffer 120 at a faster rate than the samples that are input into interpolator 110 from sample buffer 105.
Considering a single raised cosine pulse without noise, the early and late samples should have the same magnitude, if the on-time value is at the peak of the pulse. However, if the on-time value is earlier than the peak time, the early sample magnitude should be smaller than the late sample magnitude. Similarly, if the on-time value is later than the peak time, the early sample magnitude should be larger than the late sample magnitude.
Tracker 100 separately calculates metrics of a frame. A metric is an accumulated signal in a frame that is representative of the early or late samples. L2s shown in accumulators 122 and 124 of FIG. 1 represent squared L2 metrics for a corresponding early sample and late sample. For reasons of clarity, demodulation, despreading and unscrambling processes are not shown herein FIG. 1, but are included in the calculations performed at accumulators 122 and 124 in FIG. 1.
A difference (d) between the early and late metrics in a frame is calculated and accumulated in a calculator 130 as a variable D. At the end of each frame, D is compared in a comparator 140 with a fixed threshold value (thresh). If the absolute value of D is larger than thresh, which means the error is significant, a first error correction unit 152 in adjuster 150 performs an error correction by adjusting the on-time error value by plus or minus 1/16 chip period (Tc), depending on the sign (+ or −) of D. A second correction unit 154 reduces the magnitude of D by the threshold value thresh. The adjustments to on-time errors are then fed back from adjuster 150 to both the sample buffer 105 and interpolator 110 for a subsequent data frame.
The IS-95 standard algorithm described above for tracker 100 has been shown to be a very good performer in the field. However, for a system operating according to the third-generation UMTS standard, this design is inadequate for two reasons. First, receivers such as RAKE receivers in a UMTS need to cover a much wider range of signal-to-noise ratios than other communication systems, due to the various data rates supported by the standard. Thus, the expected values of d and D in FIG. 1 apparently increase as signal power increases. Accordingly, the threshold value should also increase as the signal-to-noise ratio increases.
Additionally, the threshold value should be adapted to handle different slot formats and compressed mode patterns, which are design considerations unique to UMTS systems, since all of these factors change the statistical properties of d and D. Therefore, in order to cover all transmission scenarios, a large number of thresholds need to be generated and stored in a lookup table. This process of generating each threshold individually and selecting a threshold on the fly requires a complicated and costly tracker design.
A second problem with the conventional IS-95 tracker is with the accumulation of d, the difference value between early and late metrics. In a UMTS standard, one transmission may include more than one slot format for different frames. If metrics calculated from different slot formats are accumulated together, the thresholds designed for a particular slot format may not work adequately, and tracker performance may therefore degrade significantly.